The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A cable tensioning system is often needed for use in tensioning high-strength cables of the “endless loop” variety. In essence, these cables consist of two end fittings (often called “thimbles”) with many wraps of a single yarn (or a few yarns) around the thimbles forming a cable of essentially unidirectional fiber loops. The single pair (or few pairs) of fiber ends may be tied together or left loose. In the latter case, friction among the fibers is sufficient to prevent the loops from opening up. Such cables can achieve a very high fraction of the native fiber strength and rigidity, making them attractive for applications in which high strength, high rigidity and low weight are important.
The native fiber strength of such cables can be very high compared to the strength of typical structural materials used in both compression and tension. For example, the DuPont Corporation's aramid fiber KEVLAR® has a tension yield strength of 525,000 PSI. Used with a safety margin and a factor for losses at the thimble, its limit load could be as high as 330,000 PSI. This is more than eight times greater than the limit of stress levels of aircraft structure, which is typically around 40,000 PSI. These cable materials also have much greater specific strength than metal used in cables. For instance, at yield, Kevlar has a strength of 525,000 PSI with a density (relative to water) of 1.44. Steel used in cables has a strength of around 300,000 PSI but a density of 7.8. On a pound-for-pound basis, Kevlar is more than nine times as strong as high-strength steel wire. Thus, cable bracing via high-strength fibers can be very attractive when weight is an important concern. However, the use of high-strength synthetic cable demands relatively large termination thimbles to avoid “pinching” the fibers as they wrap around the thimble. This is an important factor that must be addressed in any tensioning system used with such high strength synthetic cables.
It is a characteristic of a cable that it is capable of acting only along its longitudinal axis in tension. In compression, cable simply buckles out of the way and exerts no significant force. Similarly, a cable does not carry significant bending or torsional load; it only works in tension.
Cables are often used as elements in structures to brace or support rigid elements. An alternative to cable bracing is the use of a rigid brace that operates in tension when bracing in one direction and compression when bracing in the opposite direction. Because cable cannot brace in the compression direction, it is typical to use two cables in place of a single rigid brace. These cables are generally set in opposition so that force on the structure in one direction tends to tension only one cable while force in the other direction tends to tension only the other cable.
When set in opposition as described above, it is possible to pre-tension the cables so that both cables experience tension even when there is no external load on the structure. Such pre-tension tends to load other elements of the structure in compression.
Pre-tension of the cables tends to increase the effective rigidity of the bracing. This rigidity may be expressed in terms of inches of deflection-per-pound force exerted or degrees of angular deflection-per-pound of force or foot-pounds of torque. Cable pre-tension tends to increase this rigidity by a factor of two over the range of force during which both cables remain under tension. As the load is increased, a point is reached at which one cable becomes slack. At this point, the bracing rigidity returns to the same value as in an un-pre-tensioned arrangement. From the above, it will be appreciated that it is often desirable to pre-tension bracing cables in order to increase rigidity.
Installation of braces into a structure can pose an additional challenge. In a structure with precise dimensions it may be possible to make a cable that fits the structure perfectly so that it may be installed with no slack and no pre-tension. However, if the cable is slightly shorter than the distance between the mounting points, the cable may be very difficult to install due to the possible rigidity of the structural elements. For this reason, it may be advantageous to make the cable slightly longer than the expected distance and to take up the slack with a tensioning mechanism. Cable slack is very undesirable because it gives zero bracing rigidity to the structure over the load range where both cables are slack. Having cable slack is worse than having no pre-tension on the cable. However, it is not easy or desirable to install a cable that is shorter than the mounting distance. That is, it is not simple or desirable to stretch the cable before assembly so that it is tensioned after installation.
From the above, the following summary can be presented:
the problem is particularly significant when using high-performance bracing cable that is relatively lightweight, strong and rigid;
pre-tensioning of the cable is advantageous because it increases bracing rigidity by eliminating slack and by engaging both cables in the bracing action; and
tensioning the cable before installation into the structure is generally not desirable. A corollary to this is that distorting the structure to reduce the distance between mounting points to permit assembly is also generally not desirable.
From the above, it can seen that a suitable mechanism for pre-tensioning a cable is highly desirable. Furthermore, the optimal degree of pre-tensioning that should be applied to a cable is often not straightforward to estimate. In a simple structure with a single load case, it can be shown that a pre-tension level equal to one-half of the limit tension provides extra rigidity over the full range of loads without any increase in the maximum stress levels of the structural components. However, in many structures, several load cases may be applicable. Some of these cases may not directly involve the bracing and in such cases the pre-tensioning of the bracing cables may increase the maximum load on the other components. In such cases, the optimum level of pre-tension may be determined after a comprehensive examination of all load cases with a range of alternative pre-tension levels and bracing cable cross sections. It may be that a lower pretension with a larger cross section (and heavier) cable avoids additional loads on the other components and results in a lighter structure overall while providing the necessary level of rigidity.
Another aspect of the problem is to provide a mechanism for pre-tensioning that provides an accurate level of pre-tensioning. If the pre-tensioning is too little, then bracing rigidity will drop to one-half at too low a load level. This may have adverse consequences on the behavior of the structure including excessive deflection under load or a reduced resonant frequency of the structure. If the level of pre-tensioning is too great the cables and the other structural elements may be overstressed at the limit load. Thus, a mechanism that provides an accurate degree of pre-tension is desired. Furthermore, it would be advantageous for this mechanism to be “fool-proof” in the sense that it's construction makes it difficult for an individual to install incorrectly.
Related to the need for an accurate level of pre-tensioning is the need for a range of pre-tensioning distances. The degree of pretension (in terms of stress level) is directly related to the length of the cable, the elastic modulus of the cable, and the amount of slack built into the cable length. This means that the tensioning mechanism design concept is preferably adaptable to a range of tensioning distances even if each individual tensioning device is designed for a single pre-tension distance.
Different cable brace applications will involve different load levels according to the loads on the structure. Thus, an effective tensioning mechanism would ideally be adaptable to a range of tensioning force and be able to accommodate a range of maximum force levels. Alternatively, if two mechanisms are used on a single cable the pre-tension distance of each mechanism can be cut in half. This may result in a more compact tensioning mechanism. Also, it is desirable for the pre-tensioning mechanism to be easy to install and operate.
With a tensioning mechanism, being compact and lightweight can be of particular importance. Compact size may be desirable in that the physical size of the tensioning mechanism may influence the size and weight of other structural elements; i.e., small size can lead to a compact, lightweight connecting structure. An aspect of this is the need to integrate well with the end thimbles of the cable. These thimbles are generally cylindrical in form with a diameter and length determined by the cable load and the fiber properties. This is described in detail in U.S. patent application Ser. No. 11/332,907, filed 17 Jan. 2006, entitled “Cable Termination with Nested Thimbles”. Thus, it will be appreciated that the product of thimble diameter and width is approximately constant for a given maximum load and fiber type. A narrow thimble must be large in diameter and vice versa. For some high-performance synthetic fibers, the thimbles must be unexpectedly large, much larger than for metallic cables. These large thimbles still need to integrate well with the tensioning mechanism.
Various forms of tensioning devices have been developed in attempts to provide mechanisms for applying a pretension to a cable. One such device is the well known turnbuckle. Turnbuckles are commonly used for bracing cables. A tensioning screw is another device used for this purpose. Such a device is shown in FIG. 1. A cable termination can be made in which a screw is used to tension a cable. For example, a U-shaped fitting can capture the cable thimble in its open end. The thimble can be pinned or bolted to the fitting. A half-cylinder block can fill in the bottom of the U-shaped fitting. A screw (or multiple screws) through this half-cylinder and fitting can attach the fitting to fixed structure. Screwing the screw into the fixed structure can also pre-tension the cable. This device is in essence one-half of a turnbuckle.
A Spanish windlass is another type of well known tensioning system. The Spanish windlass is used to tension a loop of cable or rope. It is simply a stick placed within the middle of loop and rotated about the longitudinal axis of the loop. This twists the loop about itself, increasing the path length of the loop and thereby its tension. The Spanish windlass tends to be used in relatively impromptu applications.
Still another type of tension mechanism is an over-center device. Such mechanisms make use of an over-center lever to tension a cable. An hydraulic ram is yet another form of tensioning mechanism. A hydraulic ram that is placed in series with a cable enables a pressure in a cylinder of the ram to exert tension on the cable. This can be controlled so as to provide a specified level of pre-tension force or a specified linear displacement of the cable. Such devices are used on some sailboats to control the tension (length) of the cable between the transom and upper mast.
Still another form of tensioning mechanism is a shaft that extends from an eccentric. Such devices are sometimes used with bicycles in which the pedal crankshaft runs through bearings that are mounted off-center in a cylindrical housing. This housing fits within a shell and may be turned and locked at a range of angular orientations. A variation of angle tends to move the crankshaft fore and aft, providing a variation in the distance between the crank sprocket and the driven sprocket so as to adjust the tension or slack in the chain.
All of the above described, prior-developed tensioning mechanisms have drawbacks, as listed below:
Turnbuckle:                adds at least one major connection by fitting between the structure and the cable end—both ends of the turnbuckle must be attached;        replaces some length of the lightweight cable with the equivalent of an internally threaded (female) cylinder and two externally threaded screws, which adds significant weight;        screw connections in tension require large screws to account for the high thread stresses and high stresses in the core of the threaded portion of the screw; this adds significant weight;        infinite adjustment; it is possible to under or over-tighten a turnbuckle connection.        difficult integration between the turnbuckle and a large cable thimble.        
Tensioning Screw:                Heavy; screw connections in tension require large screws to account for the high thread stresses and high stresses in the core of the threaded portion of the screw;        possibly awkward assembly; the fitting tends to shroud the screw head, possibly making it difficult to get a wrench on the screw and to tighten it;        infinite adjustment; it is possible to under-tighten the screw connection.        
Spanish Windlass:                unsuited in many applications because it involves twisting the cable, which can impart undesired stress on the cable fibers by altering the relative path lengths and angles of individual fibers in the cable; this can reduce the strength of the cable;        twisting the cable tends to reduce the rigidity of the cable, an undesirable trait;        the windlass “stick” must be fixed to some structure to prevent un-winding; this is likely to require an additional structure for this purpose.        infinite adjustment; it is difficult to know when the correct degree of pre-tension has been achieved; it is easy to under or over-tension the cable.        
Over-Center Device:                difficult to make most over-center devices work with a large cable thimble; the cable and/or the cable thimble tends to conflict with the shaft about which the lever arm rotates.        
Hydraulic Ram:                complex and costly.        a failure in the hydraulic system could result in loss of pre-tension; alternatively, a locking pin could be used to fix the hydraulic piston, but such a pin would need to be large and likely would be heavy.        typical high-performance hydraulic system pressures are on the order of 5000 PSI; typical cable limit tensions may be greater than 200,000 PSI; as such, the piston diameter would be more than six times as large as the cable diameter if it is to fully pre-tension the cable.        
Shaft in an Eccentric:                the shaft is moved fore and aft relative to the fixed structure and the chain wheel (sprocket) is located on one side (asymmetrically) of the fixed structure; this places asymmetrical (and higher) loads on the structure resulting in a heavier structure and mechanism.        
In view of the foregoing, it will be appreciated that there is a need for a robust yet compact tensioning system that is well adapted for use in a variety of applications to apply a pretension force to a high strength cable, and that enables a precise amount of pretension force to be applied to a cable.